While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as light-emitting materials, sometimes referred to as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes. In particular, there is a need to be able to adjust the emission wavelength of the light-emitting material for various applications. For example, in addition to the need for blue, green, and red light-emitting materials there is a need for blue-green, yellow and orange light-emitting materials in order to formulate white-light emitting electroluminescent devices. For example, a device can emit white light by emitting a combination of colors, such as blue-green light and red light or a combination of blue light and yellow light.
The preferred spectrum and precise color of a white EL device will depend on the application for which it is intended. For example, if a particular application requires light that is to be perceived as white without subsequent processing that alters the color perceived by a viewer, it is desirable that the light emitted by the EL device have 1931 Commission International d'Eclairage (CIE) chromaticity coordinates, (CIEx, CIEy), of about (0.33, 0.33). For other applications, particularly applications in which the light emitted by the EL device is subjected to further processing that alters its perceived color, it can be satisfactory or even desirable for the light that is emitted by the EL device to be off-white, for example bluish white, greenish white, yellowish white, or reddish white.
White EL devices can be used with color filters in full-color display devices. They can also be used with color filters in other multicolor or functional-color display devices. White EL devices for use in such display devices are easy to manufacture, and they produce reliable white light in each pixel of the displays. Although the OLEDs are referred to as white, they can appear white or off-white, for this application, the CIE coordinates of the light emitted by the OLED are less important than the requirement that the spectral components passed by each of the color filters be present with sufficient intensity in that light. Thus there is a need for new materials that provide high luminance intensity for use in white OLED devices. The devices must also have good stability in long-term operation. That is, as the devices are operated for extended periods of time, the luminance of the devices should decrease as little as possible.
Rigidized boron complexes have been used as labeling dyes in analytical and biological applications; for example, see U.S. Pat. No. 4,774,339, EP 747,448 and EP 46, 861. However, boron complexes have found only limited application as dopants in electroluminescent devices. In one example, a useful class of dopants is derived from the 5,6,5-tricyclic pyrromethene-BF2 complexes and disclosed in U.S. Pat. No. 5,683,823; JP 09 289,081A; and JP 11 097,180A. These materials are characterized by typically narrow emission spectra, which may result in attractively high color purity. However, the green emitting unsubstituted or alkyl substituted pyrromethene-BF2 complexes exhibit relatively low quantum efficiencies of electroluminescence. In order to achieve highly efficient OLEDs, one needs to use phenyl rings as substituents thereby extending the conjugated π-system. As a result, the emission wavelength typically becomes red-shifted yielding a reddish amber color, which is the shortest wavelength light that can be emitted by pyrromethene-BF2 complexes with good efficiency. In simple terms, luminance efficient green or blue-green OLEDs do not appear to be conveniently obtained with pyrromethene BF2 complexes used as dopants.
JP 2001294851A describes boron complexes of heterocycles used in an electroluminescent device. For example, it describes materials in which one ring includes a cyclic amide or sulfonamide. However, these materials are reported to afford a narrow emission spectrum, which would not necessarily be desirable for use in a white-light-emitting device, which must emit a broad spectrum of light. Also, in some cases these materials were not very efficient at emitting light.
US 2003/0198829A1 and US 2003/0201415A describe electroluminescent (EL) device containing a boron dopant compound containing a bis(azinyl)methene boron complex. Such compounds, however, can be inefficient in their quantum efficiency of emission. The use of these materials is also limited by the difficulty in tuning the emission wavelengths, for example, to obtain a blue-green emission.
Bis(phenyloxy)pyridine borohalide complexes have been described by J. Feng, Y Liu, F Li; Y. Wang, S. Liu, Syn. Met., 137, 1101, (2003), J. Feng, Y Liu, F Li; Y. Wang, S. Liu, Optical and Quantum Electronics, 35, 259 (2003), P. Chou, C. Pi-Tai; C. Cheng, C. Chiou, G. Wu, Angew. Chem., Int. Ed., 41, 2274, (2002), J. Feng, Y. Liu, Y. Wang, S. Liu, Chinese Journal of Luminescence, 23, 25 (2002), Y. Liu, J. Guo, H. Zhang, Huidong; Y. Wang, Angew. Chem., Int. Ed., 41, 182, (2002), Y. Li, Y Liu, W. Bu, J. Guo and Y. Wang, Chem. Comm. (Cambridge), 16, 1551, (2000), and Y. Wang, Y. Wu, L. Yanqin, L.Yu; Lu, J. Shen, Chinese Patent publication CN 1245822A. K. Ueno, K. Suzuki, A. Senoo, H. Tanabe, S. Yogi, EP 1,138,683A2, describe bis(aryloxy)pyridine boroaryl complexes.
Borondiketonates are an interesting class of materials that have been described in JP 2000/030869 as useful in the electron-transporting layer of an OLED device. They have also been reported as light-emitting materials in JP 2000/159777 where they are referred to as dioxoborane compounds. The materials are shown as the sole emitting layer component or as a dopant in an AlQ host. However, these materials, when used with a host other than a hydrocarbon, such as tris(8-quinolinolato)aluminum (III) (Alq) or 4,4′-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), tend to give unsatisfactory luminance and/or color.
Thus there remains a need for organic EL device components that will provide good color and high luminance.